MULTIWAVELENGTH STUDY OF PHOTODISSOCIATION REGIONS AND A NEAR-INFRARED !MAGER BY .ROLAINE C. YOUNG OWL. B.S., University of California, Los Angeles, 1985 M.S., University of Illinois at Urbana..:Champaign, 1995 THESIS Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Astronomy in the Graduate College of the . University of Illinois at Urbana-Champaign, 1999 Urbana, Illinois Q .~ ~~.6'c?.3 T..J6- CJ .!:J UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN THE GRADUATE COLLEGE March 31, 1999 (date) WE HEREBY RECOMMEND THAT THE THESIS BY Rolaine C. Young Owl ENTITLED·__ M_u_lt_i_w_a_v_e_l_e_n_gt_h_S_u_d_y_o_f_P_h_o_t_o_d_is_s~·o_c_i_a_t_io_n_R_e_g_i_o_n_s_a_n_d_a_~ Near-Infrared Imager BE ACCEPTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF___ D_o_c_t_o_r_o_f_P_h_i_l.;;_o_s-'-o_._p.;..chy,___ _____________ _,#'~~ Margaret M. Meixner Director of Thesis Research Richard M. Crutcher Head of Department Susan A. Lamb Lewis E. Snyder t Required for doctor's degree but not for master's. 0-517 UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN GRADUATE COLLEGE DEPARTMENTAL FORMAT APPROVAL THIS IS TO CERTIFY THAT THE FORMAT AND QUALITI OF PRESENTATION OF THE THESIS SUBMITIED BY Rolaine C. Young Owl , AS ONE OF THE REQUIREMENTS FOR THE DEGREE OF__ D_o_c_to_r~o_f~P~h_i_l~os_o_p_h_Y __________ ARE ACCEPTABLE TO THE Department of Astronomy FullName Department. Division or Unit 17_ l/w. -<ftt Date of Approval Departmental Representative Abstract In this thesis, observations of photodissociation regions (PDRs) in the millimeter, far­ infrared, and near-infrared wavelength regimes are interpreted in the context of PDR theory. Three modifications to the standard Tielens & Hollenbach PDR model are considered. The , first modification involves adding nitrogen chemistry to the TH model (Wolfire & Tielens). The second modification extends the TH model to lower-density and lower-excitation con­ ditions (Hollenbach, Takahashi & Tielens). The third accounts for the effects of the color temperature of the illuminating field (Spaans, Tielens, van Dishoeck & Bakes). Chapter 1 is an introduction to this work and presents the motivation for the multiwavelength approach. In Chapter 2, high-resolution millimeter-wavelength maps of the high-density PDR in the Orion Bar were used to evaluate the PDR nitrogen chemistry model calculated by Tielens & Wolfire. Besides demonstrating the morphology of the molecular gas in the Orion Bar, we found that the production of HCN and HCO+ occurs mainly in dense clumps near the PDR surface. Chapter 3 focuses on the energetics of lower-density PDRs. We describe the analysis of far-infrared continuum and fine-structure line emission from the PDRs in several reflection nebulae. We found that the observed. line intensities and predicted densities fall within the regime defined by the low-density model of Hollenbach, Takahashi & Tielens. We also found that varying the stellar type of the illuminating source has little effect on the heating efficiency of the PDR gas, which agrees with the color temperature model of Spaans, Tielens, van Dishoeck & Bakes. Chapter 4 describes the construction and operation of the Near-Infrared lmager (NIRIM), which was commissioned at the Mt. Laguna 1-meter telescope in August 1995. Also described is the use of the NIRIM to obtain high-resolution iii wide-field images of the PDR in the reflection nebula NGC 2023. We found that the fluores­ cent molecular hydrogen in NGC 2023 has a filamentary structure, whereas recombination emission from neutral atomic carbon appears much more diffuse. iv To my parents, Roland and Pearl v Acknowledgements First of all, I would like to thank my Ph.D. thesis advisor, Margaret Meixner for her excellent guidance, for her generosity, honesty and encouragement, and for introducing me to some topnotch collaborators. Among these are Mark Wolfire and Xander Tielens, who wrote the code and didthe calculations for the PDR nitrogen chemistry model in this thesis; Mike Haas and Alexander Rudolph who obtained and reduced the far-infrared spectra of the reflection nebulae; and Bob Leach who collaborated with Margaret and me on the near-infrared camera project. I would also like to thank Jan Tauber who offered invaluable insight and advice about BIMA data reduction. I would like to thank my University of Illinois colleagues for their support and encour­ agement. I would especially like to thank David Fong, who gave me invaluable help with the FIR analysis; Rob Klinger, for helping me with some of the figures when LaTex proved intractable; and Ramprasad Rao for helping me through the final struggle to get this thesis into proper thesis format. I would like to thank undergraduates Tony Marcotte and Jason Davis for helping me with the NIR data reduction. Thank you to Professors John and Lanie Dickel for their kind generosity in allowing me to stay at their home while finishing my thesis. This thesis could not have been written without the help of these extraordinary people. And finally, I would like to thank my family, especially my husband Marcus Young Owl for his patience during the months we had to live apart. Thank you to my parents, Pearl and Roland Chandler, and to my four wonderful sisters Avis Sutton, Adrienne Parker, Gayle Kaneaster and Delia L. Chandler. Thank you for your unwavering support of my dreams and aspirations, none of which would have been realized without your unconditional love. Vl Table of Contents Chapter 1 Introduction ................. 1 Chapter 2 HCN and HCQ+ Images of the Orion Bar . 7 2.1 Introduction . 7 2.2 Observations . 10 2.3 Results . 14 2.3.1 HCO+ and HCN maps 14 2.3.2 Crosscut scans . 15 2.3.3 Column densities . 22 2.4 PDR Theory . 24 2.4.1 Interclump Medium . 27 2.4.2 Clumps . 33 2.5 Discussion . 35 2.5.1 Comparison of Model and Observations . 35 2.5.2 Differences in Hco+ and HCN 42 2.6 Conclusions ................... 44 Chapter 3 Far-Infrared Spectroscopy of Low-Excitation Photodissociation Regions in Several Reflection Nebulae. 46 3.1 Introduction . 46 3.2 Observations and Results . 48 3.3 Physical Conditions T 0 , n 0 , G0 , and E 53 3.3.1 Heating Efficiency . 53 3.3.2 Excitation temperatures . 57 3.3.3 Detailed Balance . 58 3.3.4 Physical conditions in reflection nebula PDRs 62 3.4 The Geometry of the NGC 1977 PDR: [OI] 63µm scan 66 3.5 Testing low-excitation PDR theory with the observations 69 3.5.1 Heating efficiency vs. effective temperature . 70 3.5.2 Observed line intensity ratios vs. G0 72 3.5.3 Dust temperature vs. G 0 73 3.6 Conclusions . 77 vu Chapter 4 NIRIM: a Dual Purpose Near Infrared (0.76-2.5µm ) Imaging Camera for Wide Field and High Resolution Imaging . 79 4.1 Introduction . 79 4.2 Optical Designs . 80 4.3 Cryostat Design . 85 4.4 Electronics . 86 4.4.1 Array Detector 86 4.4.2 Controller and Signal Processing Electronics . 87 4.4.3 System Gain . 88 4.4.4 Readout scheme . 90 4.5 Performance . 90 4.6 NIRIM Images of the PDR in NGC 2023 92 4.6.l Observations 92 4.6.2 Results . 93 Appendix A PDR Model Chemistry . ...... 103 A. l Thermal Processes 103 A.2 Chemistry . 104 A.3 HCN Line Transfer 104 References . 105 Vita .... 113 Vlll List of Tables 2.1 Orion Bar Crosscut Results 22 2.2 Species Parameters 23 2.3 Interclump Model Results 33 2.4 Orion Bar: Observed · and Predicted I and Nr 36 3.1 Target Reflection Nebulae for FIR Spectroscopy 49 3.2 KAO Observations . 51 3.3 FIR Line Intensities .............. 59 3.4 NGC 1977 [OI] 63µm Cross-Cut . .. 60 3.5 Results of FIR Analysis on Reflection Nebulae 61 3.6 Estimated Densities ........ 66 4.1 Optical Design: NIRIM at MLO 1 m 82 4.2 Table 4.1 (cont'd) ........ 83 4.3 Optical Design: NIRIM at UNISIS 84 4.4 Characteristics of NIRIM . 86 4.5 NIRIM Performance in Broad Band Filters at MLO 1 m 91 4.6 Performance of NIRIM in Narrow Band Filters at MLO 1 m 92 IX List of Figures 1.1 In a massive star-forming region, a PDR is intermediate between the HII ! I region and the molecular cloud. 2 1.2 Chemical stratification in PDR gas is illustrated by the standard 1-D model. 4 It ' 2.1 Channel maps of the Orion Bar in the J=l-0 line emission of HCO+ at ! 89.188518 GHz are made from combined single dish and interferometry data. 12 2.2 Channel maps of the Orion Bar in the J=l-0 F=2-1 emission line of HCN I at 88.631847 GHz are made from combined single dish and interferometry data. 13 2.3 The Orion Bar in all three hfs components of the HCN J=l-0 triplet is shown. 16 I 2.4 Presented here is an integrated flux map of the Orion Bar in HCO+, in the 1 velocity range 9-12 km s- . 18 I 2.5 This HCN integrated flux map has the same mapping parameters as in Fig- i ure 2.4.. 19 2.6 The HCN integrated flux map is used as a reference map. 20 I 2.7 The relative intensities of several molecular species are plotted as a function of distance from the IF. 21 I 2.8 The contours of an integated flux map of the Orion Bar in 13 CO J=l-0 line I' emission is overlaid on the greyscale HCO+ map. 25 2.9 The contours of an integated flux map of the Orion Bar in 13 CO J=l-0 line l emission is overlaid on the greyscale HCN map.